Redundant fieldbus system

ABSTRACT

A redundant fieldbus redundant system includes two independent conditioned power modules which automatically detect cable faults, such as short or open circuits on both the host and field sides of the network. The power modules are interfaced directly on one side to the primary and backup H1 cards of the host system, and are directly interfaced on the other side to an automatically terminating network device coupler module which provides connections to field devices. The redundant fieldbus system provides power and communications in a parallel physical configuration between the host system and the field devices irrespective of any single point failure in the network. In case of a fault, the redundant fieldbus system automatically eliminates the faulty section of the network, switches power and communications to the healthy portion of the network and terminates the network for signal integrity.

RELATED APPLICATION

This application claims priority from U.S. patent application Ser. No.11/351,144, filed Feb. 8, 2006, which application is incorporated byreference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to an automation or control network suchas a fieldbus network that facilitates an extended level of redundancysuch as redundancy within the power distribution facilities of afieldbus network.

2. Description of the Related Art

Fieldbus networks are advantageously used in industrial control systemsand particularly in industrial control systems that provide distributedcontrol incorporating field devices having local processors. A fieldbusnetwork is a two-wire network capable of delivering DC power to remotelyinstalled field devices and capable of providing bidirectional digitalcommunications between the remote field devices and the host system. Thedigital communications are typically carried on a 31.25 kHz carriersignal in the standard-defined H1 fieldbus network. Various fielddevices might be attached to be powered from and communicate over thenetwork, including controllers, actuators and sensors. Multiple fielddevices can be attached to one fieldbus segment. The fieldbus istypically implemented on the physical level as two wire shielded cable.More details about conventional aspects and implementations of fieldbusnetworks can be found in the International Electrotechnical Commissionstandard IEC 61158-2 which is specifically for industrial networks andpromoted by the FOUNDATION Fieldbus and PROFIBUS organizations.

In the host system side of the fieldbus network, primary and backup H1cards are mounted as front end interface modules to the networked fielddevices to provide continuous communications. The backup H1 cardprovides redundancy and takes over the communications between the hostand the field devices in case the primary H1 card fails due to anyreason. Each H1 card might be configured to provide more than onechannel or more than one segment per card; it depends on themanufacturer.

For simplicity, the following discussion refers to one fieldbus segmentper H1 card, but implementations can readily provide more segments percard. In a conventional system, the output connection of the primary andthe backup H1 cards can be linked together at the host system so thatone single two-wire cable can be interfaced directly between theconditioned power modules and the H1 cards. In some otherimplementations, both H1 cards can be wired individually to theconditioned power modules so that the common link between the cables ismade at the power modules. The latter method is used to provide anadditional cable redundancy between the H1 cards and the conditionedpower modules.

The conditioned power modules also provide DC power to remote fielddevices via a single two-wire cable connected to an electronic devicecoupler. Some types of conditioned power modules are designed with aredundancy feature in a similar fashion to H1 cards. When twoconditioned power supplies with this redundancy feature are connected inparallel to the networked field devices, both power supply modulesoperate together and share the same load. If either of the power supplymodules fails, the remaining, healthy power supply module will supplythe extra power to the load (i.e., the field devices).

In a fieldbus network, the communication signals should be terminated atboth ends of the network cable. A matching terminator circuit is fittedat the front end of the power module, and another matching terminatorcircuit is mounted at the last node of the network. A typical networkmay have multiple drop or spur connections that each interface to one ormore local field devices.

FIG. 1 is a schematic circuit diagram that illustrates the common methodof installation recommended by the IEC 61158-2 standard. FIG. 1 shows afieldbus network 10 in which all field devices D1, Dn (n could be up to32 devices per segment) are linked to the host through primary H1 card12 and backup H1 card 14 and through the single cable 16. Cable 16 andthe communication signals are terminated at both ends of the cablenetwork by terminators T1 and T2. Primary conditioned power supplymodule 8 and secondary conditioned power supply module 20 are connectedin parallel to bus 16 so as to share the load of the field devices D1,Dn. As illustrated, field devices may be connected to the network cable16 through a device coupler 22. Typical device couplers 22 provide astandardized interface that allows for easier connection of spurs orindividual field devices to the network cable 16. Some device couplersprovide fuses or current limiting technology to address local faults andto provide local fault indicators. One of the restricting factors inFOUNDATION fieldbus technology is that the physical layer used for theH1 network does not naturally allow for redundancy. This lack ofredundancy may in part lead to undesirable expense and down time for thefieldbus network.

SUMMARY OF THE PREFERRED EMBODIMENTS

A particularly preferred embodiment of the present invention provides afieldbus network comprising first and second power supplies. The firstpower supply provides independent host and field power supply paths to ahost side terminal and to a field side terminal, respectively. The firstpower supply has a first fault detector circuit coupled to host andfield switches adapted to selectively decouple the host and field powersupply paths in response to detection of a fault on host and fieldsides, respectively. The second power supply provides independent hostand field power supply paths to a host side terminal and a field sideterminal, respectively. The second power supply has a second faultdetector circuit coupled to host and field switches adapted toselectively decouple the host and field power supply paths in responsedetection of a fault on host and field sides, respectively. The fieldbusnetwork includes first and second fieldbus cables coupled in parallel toa field device. The first fieldbus cable is coupled between the fieldside terminal of the first power supply and the field device. The secondfieldbus cable is coupled between the field side terminal of the secondpower supply and the field device.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention are illustrated in the attacheddrawings and can be better understood by reference to those drawings inconjunction with the detailed description. The attached drawings form apart of the disclosure.

FIG. 1 schematically illustrates a conventional configuration of afieldbus system according to the IEC 61158-2 standard.

FIG. 2 schematically illustrates a preferred configuration of aredundant fieldbus network according to the present invention.

FIG. 3 schematically illustrates another preferred configuration of aredundant fieldbus network according to the present invention.

FIG. 4 schematically illustrates a preferred conditioned power supplymodule that can be used advantageously in either the FIG. 2 network orthe FIG. 3 network.

FIG. 5 schematically illustrates a preferred implementation of a devicecoupler that can be used advantageously in either the FIG. 2 network orthe FIG. 3 network.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The typical fieldbus system is robust. Unfortunately, the typicalfieldbus system has a weakness in that the system uses a single powercable. The power and the communications signals for the fieldbus dependon the integrity of that single, twisted-pair cable between the host andthe remote field devices. In conventional implementations, any cablefault will result in the unavailability of not only any associated fielddevice but of all field devices on that network. As a result, the systemloses its control and communications with the field devices. To improvethe reliability, availability and safety of fieldbus systems, thenetwork cable needs to be fully protected and maintained. Alternately, afieldbus system may be made completely and automatically redundant bysupplying the network with cable fault detection and automatic signaltermination.

Preferred aspects of the invention can be combined to provide completefieldbus network redundancy from the front end interface of the H1 cardsto the field devices, preserving network availability and continuouscontrol over the process without user intervention. Preferred aspects ofthe present invention allow the fieldbus network to provide redundantcable runs that are independently available to the field devices. Otheraspects of the invention can be used to automatically terminate thenetwork cable to preserve signal integrity in the field if a power lossoccurs on either the host or field side of the connection network. Anydetected cable fault is preferably reported to the user, and the systempreferably automatically directs communication signals to the healthy(no cable fault) path of the network to maintain the process undercontrol. Preferred implementations of the system allow hot swapping offaulty items, such as power modules, H1 cards and cables without anydowntime in the process associated with the fieldbus network. Preferredembodiments of the invention also offer a high degree of safety tocritical application loops and full access to uninterruptible continuousprocess measurement.

FIG. 2 shows a schematic diagram of a preferred implementation of aredundant fieldbus system 30. In the FIG. 2 system, a primary H1 card 32and a backup H1 card 34 within a redundant pair are connected torespective legs of the segment, fed into respective conditioned powersupplies 36, 38 and wired out into the field. The primary and backup H1cards provide the interface to the host control system 39. Each outletcable 40, 42 is wired to a common device coupler 44 located close to thefield devices D1, D2 and Dn. Here again, n represents the number offield devices such as controllers, actuators or sensors, and can be upto, for example, 32 field devices per segment. Here, the illustratedcables are any cables appropriate to the fieldbus network and aretypically two-wire cables. The device coupler 44 is then wired tovarious field devices D1, D2 and Dn required by the process associatedwith the network 30. Segment termination T1, T2 is provided at the frontend of each conditioned power supply 36, 38, so that the fieldbusnetwork is fully terminated at each end. The illustrated networkconfiguration provides a primary and redundant interface card,conditioned power supply and cable to the device coupler 44 so that thedevice coupler has a complete set of redundant facilities. This ensuresthat the device coupler 44 can provide power and communications to itsconnected field devices over independent paths.

Redundancy can also be achieved as shown in FIG. 3. Note that thecomponents of the illustrated FIG. 3 fieldbus network are generally thesame as are illustrated in the FIG. 2 network and so these componentsare designated with the same numerals in FIGS. 2 and 3 and the abovediscussion is not repeated here. The primary 32 and the redundant 34 H1cards are connected to the conditioned power modules, 36, 38 (primaryand standby modules respectively), and a hardwire link 46 is madebetween the power modules 36, 38. Each outlet cable 40, 42 is wired to acommon device coupler 44 located close to the field devices D1, D2 andDn. Power and communications are provided via one cable at a time, whilethe other cable is kept in a standby state. If a cable fault occursalong the primary field cable 40, the standby power module 38 will takeover to supply power to the device coupler 44 via cable 42 so that allfield devices are kept powered. The process operates in the oppositeorder if the cable 42 has been made the primary cable and then suffers afailure, the conditioned power supply 36 will take over to supply powerto the device coupler 44 over the cable 40 to keep the field devicespowered. Front end segment terminator T1 is located at the power modulesor at H1 cards, while the other segment terminator T2 is positionedinside the device coupler 44.

Each of the conditioned power supply modules 36, 38 preferably used inthe FIG. 2 or FIG. 3 network most preferably incorporates an electroniccircuit 50 like that shown in FIG. 4. Each of the preferred power supplymodules 36, 38 (FIGS. 2, 3) delivers power to both the host system sideand the remote field devices via two independent cables, 52 and 54,respectively. Note here that FIG. 2 schematically shows a continuouscable 40, 42 connecting the H1 interface card 32, 34 to the conditionedpower supply module 36, 38 and through the device coupler 44 to thefield devices D1. D2 and Dn. The actual implementation that is presentlypreferred for use in the FIGS. 2 and 3 networks is as illustrated inFIG. 4, where the field side cable 54 connects to the host side cable 52through an inline noise filter 56. with the field and host side cablespowered through substantially independent paths, as shown in FIG. 4.

Filter 56 most preferably couples the bidirectional communicationsignals between the cables 52 and 54 and suppresses high frequency noisethat might be generated on the field side of the network. Power issupplied from module 50 (or 36 and 38 in FIGS. 2, 3) over thesubstantially independent paths within the FIG. 4 power module so thatnoise advantageously is not coupled to or from the host or field sidesof the network. Most preferably, the power supply 50 can independentlyisolate cable faults on either the host or field side cables. Each powersupply 50 is fitted with two electronic circuits 58, 60 that function asopen circuit detectors to detect cable discontinuities or other faultson either the host system side 52 or the field side 54. The open circuitdetectors 58, 60 measure the currents delivered to the host load and thefield load across resistors R1 and R2, respectively.

In normal operation of the FIG. 4 power supply circuit, the host andfield sides should draw minimum currents from the supply so that theopen circuit detectors measure currents at or above the threshold valuesrepresenting normal operation of a network free of cable faults. If atany time the drawn current on either the host or field side drops belowthe threshold values, the respective open circuit detector 58 or 60sends an alarm to the control logic circuit 62 indicating a cable fault.The control logic circuit 62 detects the alarm signal and determinesfrom which side (host or field) the cable connectivitv was lost. Upondetecting a loss of cable continuity or other fault, the control logiccircuit 62 releases two alarms, one local and one remote to the user.The local alarm is sent to a local LED1 that indicates the fault at theconditioned power supply module and the remote alarm is sent to the usersystem via an optically isolated device 64. The control logic circuit 62also sends a logic signal to either of the transistors Q1 or Q2 or toboth, depending on the type of cable fault, to switch off the supply toindicate the side of the faulty connection or cable.

At the moment when the cable fault is detected “as open circuit,” thepower supply unit 50 cuts off or ceases supplying power over cable 52 tothe connected H1 card so that the H1 card stops functioning. This isaccomplished by the control logic circuit 62 causing the transistor Q1or Q2 or both to become non-conducting on the side or sides of thenetwork the open circuit detectors 58, 60 indicate has a cable fault.When the transistor or transistors is turned off (non-conducting), poweris removed from the H1 card on the side of the network with the cablefault and that H1 card is disabled. Since the host controls both theprimary 32 and the backup H1 cards 34, the host knows which H1 card isdisabled and directs the communication signals via the still-functioningH1 card, presumably the backup H1 card. The host makes the backup H1card the main communication module for communications with the fielddevices when the primary H1 card receives no power or loses power. Thesecondary or redundant conditioned power supply 38 always powers thebackup H1 card and the device coupler 44 in the field so that thecommunication signals will be easily transmitted directly to the fielddevices and hence all field devices will remain under control.

Still referring to FIG. 4, short circuit detector 66 monitors the cableconditions on both the host system side and the field side for shortcircuits. The fieldbus network cable can be up to a 1000 meters long oneach side (host and field) as described by the IEC61158-2 standard. Forthe fieldbus network illustrated in FIG. 3, the cable network can be1900 meters as described in the IEC61158-2 standard. If a cable shortcircuit or other fault occurs on either side of the network cables, anexcessive current will flow through the fault and back through R3. Whenthis current exceeds the limit set by the short circuit detector 66,circuit 66 sends an alarm signal to the control logic circuit 62 andswitches off Q3 to isolate the faulty part and limit current drain. Thecontrol logic circuit 62 sends two alarm signals, one to a local LED2 toindicate a cable short circuit fault at the conditioned power supply 36,38 and the other alarm signal is sent to the user via the opticallyisolated device 64. Q3 stays open circuit as long as the cable isfaulty. Once the cable fault is removed, the short circuit detector 66switches Q3 on automatically. Thus, if a cable short circuit fault isdetected on the primary H1 card 32 side, the primary H1 card 32 losespower and consequently the host system redirects the communicationsignals through the backup H1 card 34. Communication signals are carriedover the other, healthy leg of the network as explained above.

FIG. 5 shows a device coupler 44 preferably used in the redundantfieldbus network of FIG. 2 or 3. The device coupler 44 of FIG. 5 ispreferably connected to cables 40, 42 and through those cables to powersupply modules 36, 38 as shown in FIG. 2 or 3. The device coupler asshown in FIG. 5 includes a diagnostic circuit connected to the primaryand redundant network cables to detect when power is lost from thecables connected on the primary and backup sides as power inputs. Ondetection of a loss of power from either cable, most preferably theauto-terminator circuit is automatically activated to terminate thetransmission line to avoid any distortion of the communication signals.Once the cable fault is removed, the auto-terminator preferably isautomatically switched off and the system resumes communicating via thewhole network. In this reset state the system is fully operational as aclosed loop. If a cable short circuit fault develops on either side ofthe network, then the system communication signals will be diverted tothe healthy H1 card side and transmitted via the healthy cable so thatall field devices are still powered. The auto-terminator then switcheson to allow normal function for the fieldbus devices.

FIG. 5 illustrates a schematic block diagram of the auto-terminationcircuit. Circuits 70 and 72 monitor the conditions of the input powersupplies at both sides of the cable connection terminals. Input powersupplies 74 and 76 correspond to the power provided over the cables 40,42 shown in FIG. 2 or 3. If input power supplies 74 and 76 are presentand outputting their values are above the threshold values set by Zenerdiodes ZD 1 and ZD2 (that is, there is no cable fault and the powersupplies 36, 38 are operating), then both Q1 and Q2 are on, which makesQ3 of the terminator network 78 switch off. The cable length on bothlegs does not affect the functionality of circuits 70 and 72irrespective of the voltage drops that are likely to occur due to theload (the field devices). The terminator network, circuit 78, includesan impedance matching circuit, which is a terminator resistor R1connected in series with a terminating capacitor C1. Circuit elementsD1, D2, D3, D4, D5, D6 are used for reverse polarity protection.

Once a cable fault (either a short or an open circuit) occurs on eitherside of the input supplies 74, 76 the voltage at the supplies will bebelow the threshold values of ZD1 or ZD2 or both, causing Q1 or Q2 toswitch off and causing Q3 to switch on. This establishes theauto-terminating function. Once the failed input power is restored andremains above the threshold values of ZD1 and ZD2, Q1 and Q2 switch backto their on conditions, while Q3 switches off and the auto-terminatingcircuit becomes non-conducting. An alternative auto-terminating circuitthat might be used is described in WIPO patent publication WO2005/032060, which is based on current sensing technology. WIPO patentpublication No. WO 2005/032060 is hereby incorporated by reference inits entirety and for all of its relevant teachings.

Circuit 80 provides spur short circuit protection. Circuit 80 is similarto the short circuit detector 66 discussed above with reference to FIG.4 and so is not discussed in detail here. If a short circuit occursacross the spur connection, due to a cable short circuit or a faultyfield device, the power to the faulty load will be switched off whileall the rest of the field devices continue to be powered and communicatewithout any loss of signal or any additional voltage drop across themain network cable segment. In typical implementations of a redundantfieldbus system according to the present invention, the main networkcable segment operates at a higher voltage because the current use byfaulty devices is avoided. Circuits 82 and 84, representative of n spursin this example, are similar to circuit 80 and are not describedseparately here for purposes of conciseness.

As discussed above, fieldbus networks are characterized in that theyprovide DC power to attached field devices, such as controllers,actuators and sensors, and carry bidirectional digital communicationsbetween a system controller and the various field devices attached tothe network segment. The digital communications are carried by an ACcarrier that is, in many circumstances, a 31.25 kHz carrier signal asdefined by the IEC 61158-2 standard. Particularly preferredimplementations of the present invention can be implemented inFOUNDATION Fieldbus and PROFIBUS types of networks. Additionalinformation regarding applications and configurations of advantageousfieldbus networks can be found at the websites and in the publicationsof the FOUNDATION Fieldbus and PROFIBUS organizations. Of course,successors to the present implementations of the standards and networksare anticipated and the present invention will find application in suchnetworks.

In addition, while a single device coupler 44 is illustrated inexemplary FIGS. 2 and 3, it is possible to have multiple device couplersand multiple spans of network cables connected within a particularnetwork. Different numbers of field devices may be provided, indifferent configurations, on the one or more device couplers. In otherimplementations, field devices can be attached without using devicecouplers. FIGS. 2 and 3 show modular configurations for the devicecouplers and the field devices. While this is presently preferred, it isnevertheless possible for the field devices to include aspects of thedevice couplers, such as the automatic termination circuitry describedabove.

The present invention has been described in terms of certain preferredembodiments. Those of ordinary skill in the art will appreciate thatvarious modifications and alterations could be made to the specificpreferred embodiments described here without varying from the teachingsof the present invention. Consequently, the present invention is notintended to be limited to the specific preferred embodiments describedhere but instead the present invention is to be defined by the appendedclaims.

1. A power supply module for a fieldbus network, comprising: a powersupply for supplying DC power to a host device over a host fieldbuscable and to a field device over a field side fieldbus cable; a hostside terminal to which a host side fieldbus cable can connect and afield side terminal to which a field side fieldbus cable can connect;host and field power supply paths coupling the power supply to the hostside terminal and to the field side terminal, respectively; a faultdetector circuit coupled to a host switch positioned along the hostpower supply path between the power supply and the host side terminaland to a field switch positioned along the field power supply pathbetween the power supply and the field side terminal, the host switchdecoupling the power supply from the host terminal upon detecting afault on the host side and the field switch decoupling the power supplyfrom the field side terminal in response to detection of a fault on thefield side; and a noise filter coupled along a communication pathbetween the host side terminal and the field side terminal, the noisefilter passing bidirectional communication signals between the host sideterminal and the field side terminal when no fault is present on thehost side and the field side of the power supply module.
 2. The moduleof claim 1, wherein the fault detector includes an open circuit detectorcoupled to the host side, a second open circuit detector coupled to thefield side, and a short circuit detector coupled to both the host sideand the field side.
 3. The module of claim 1, wherein the noise filterconnects the host side terminal to the field side terminal.
 4. Themodule of claim 1, wherein the fault detector includes a first opencircuit detector connected in series with the host switch along the hostpower supply path, a second open circuit detector connected in serieswith the field switch along the field supply power supply path, and ashort circuit detector coupled to both the host side terminal and thefield side terminal.
 5. The module of claim 1, wherein the host switchcomprises a first switch transistor that the fault detector circuitswitches off when a fault is detected, wherein the first switchtransistor switches on when the fault detector circuit subsequentlydetects absence of a fault.
 6. The module of claim 1, wherein the fieldswitch comprises a second switch transistor that the fault detectorcircuit switches off when a fault is detected, wherein the second switchtransistor switches on when the fault detector circuit subsequentlydetects absence of a fault.
 7. The module of claim 1, wherein the fieldswitch comprises a second switch transistor that the fault detectorcircuit switches off when a field side fault is detected, wherein thesecond switch transistor switches on when the fault detector circuitsubsequently detects absence of a field side fault.